02 March, 2021

Review: Light energy to chemical energy

Light reactions: make ATP + NADPH for Calvin cycle


Calvin cycle

Calvin cycle is a 3-step process


  • 1. Fixation: CO2 reacts with 5C molecule (RuBP)
    • catalyzed by Rubisco enzyme
    • new 6C split into 2 acids (3C each)


  • 2. ATP and NADPH turn 3C acids into sugars
    • reduction of acids


  • 3. Some 3C sugars recycled to make RuBP (5C)
    • one 3C sugar leaves to make glucose
    • recycling needs ATP

Making sugars requires several cycles

Rubisco - the enzyme that dominates carbon fixation






  • Most abundant enzyme in the world


  • Most CO2 converted into biomass is fixed by Rubisco


  • Large molecule : 16 polypeptides : 8 active sites

Rubisco: the clunky and slow carbon fixer




  • Only 3-10 reactions per second
    • limits photosynthesis


  • 20% error rate
    • gets worse at high temperatures


  • Also reacts with O2
    • O2 reaction called Photorespiration
    • uses ATP and NADPH to make CO2

C3 photosynthesis how did we get here…




  • Evolution doesn’t always create the best solution


  • Photorespiration is completely wasteful
    • 2x energy to produce the same amount of sugar than if Rubisco only reacted with CO2


  • Rubisco has evolved high affinity for CO2 (80x > O2)
    • compare to the composition of the atmosphere…

Rubisco and plant evolution: How did we get here?





  • Why is there so much Rubsico?


  • Why is it so big and slow?


  • Was it Rubisco always a cheater?

Why has Rubisco not been replaced?



  • Evolution of enzymes difficult
    • trade-offs between activity and stability
    • selection against modifying mutations


  • RubisCO → RubisC???
    • requires a decrease in fitness
    • plants are stuck with Rubisco


  • Natural selection increased affinity for CO2
    • makes it slow

Evolution tinkers with other parts of photosynthesis



  • Improved and recombined existing parts and pieces
    • work around the photorespiration issue


  • In response to environmental change
    • lower [CO2] & higher [O2]
    • high temperatures
    • water limitations (stomata)


  • Evolution of new photosynthesis pathways
    • Allowed plants to exploit drier habitats

Evolution of C4 photosynthesis





  • Calvin cycle moved to Bundle Sheath Cells surrounding leaf veins
    • chloroplasts now present



  • BSC cells are less permeable to gases
    • what does this mean for photorespiration?

C4 plants concentrate CO2 around Rubisco spatially




  • C4 mechanism separates Calvin cycle from high O2


  • New enzyme, phosphoenolpyruvate carboxlylase (PEPC)
    • fixes CO2 to PEP (no affinity for O2)
    • new 4C malate pumped to bundle sheath cells
    • malate broken down to release CO2


  • Hyper efficient with use of CO2
    • How does this impact stomata behavior?

Why is C4 photosynthesis not dominant?





  • Regeneration of PEP expensive (ATP)
    • in addition to ATP needed for Calvin cycle


  • Where is needed ATP generated?


  • Where should C4 plants exist?

In hot conditions, the benefits of reduced photorespiration likely exceed the ATP cost of moving CO2 from the mesophyll cell to the bundle-sheath cell

Evolution of CAM photosynthesis (C4 pathway w/out BSC)

CAM concentrates CO2 around Rubisco temporally




  • Stomata open at night only


  • 4C compounds produced in large quantities
    • stored when Rubisco is inactive


  • C4 vs CAM
    • substitute BSC for “night shift”

Yes we CAM!!




  • CAM evolved independently several times
    • ~16,000 species


  • Exist in arid environments or habitats
    • cacti
    • epiphytes


  • Are CAM plants fast or slow growing?

C3 vs C4 vs CAM: Know advantages/disadvantages of each

Why does this matter to you…



  • C4 plants are economically important
    • corn, sugarcane, sorghum & switchgrass


  • So are C3 crops
    • beans, rice, wheat, potatoes (temperate crops)


  • Global water use is set to triple by 2050
    • 70% to the agriculture sector
    • 50% transpired through stomatal pores